On 02/18/2015 09:13 PM, Steven Rostedt wrote: >> Here the same thing but without cmpxchg(). _If_ after an increment the >> value is negative then we take slowpath. Otherwise we have the lock. > > OK, so I need to make it so it can nest with trylock. I have to look at > the patch again because it has been a while.
I have reverted the patch and can confirm that cpufreq works again. I did some testing on vanilla and -RT: - down_read(l) + down_read(l) this triggers a lockdep warning about a possible deadlock the lock is obtained. - down_read(l) + down_read_trylock() this passes without a warning. So I think we good now. > An RW sem must not do two down_read()s on the same lock (it's fine for > a trylock if it has a fail safe for it). The reason is, the second > down_read() will block if there's a writer waiting. Thus you are > guaranteed a deadlock if you have the lock for read, a write comes in > and waits, and you grab the RW sem again, because it will block, and > the writer is waiting for the reader to release. Thus you have a > deadlock. I fully understand. However nesting is allowed according to the code in vanilla and now again in -RT. Lockdep complains properly so we should catch people doing it wrong in both trees. > I'll have to revisit this. I also need to revisit the multi readers > (although Thomas hates it, but he even admitted there's a better way to > do it. Now only if I could remember what that was ;-) Okay. For now I keep the revert since it looks sane and simple. > > Thanks, > > -- Steve Sebastian -- To unsubscribe from this list: send the line "unsubscribe linux-kernel" in the body of a message to majord...@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html Please read the FAQ at http://www.tux.org/lkml/
